Currently, mobile communication technologies are evolving to include the deployment of very high frequencies, larger carrier bandwidth, very high data rates and multiple heterogeneous layers. The future mobile networks (i.e. next generation of mobile networks) are likely to be a combination of evolved 3G technologies, 4G technologies and emerging ones with substantially new components millimeter Wave (mmW) based communications, high gain beamforming, etc. Due to the increasing demand to enhance wireless capacity and due to lack of availability of spectrum in lower frequency range (e.g. 800 MHz-3 GHz), it is envisioned that some next generation of mobile network use cases will require much wider bandwidth than what is available for existing mobile systems. Furthermore, the next generation of mobile network is targeting frequencies in a very wide range from below 1 GHz up to 100 GHz for.
Owing to large signal attenuation with respect to path loss when operating over such high frequencies and also the possibly much higher data demanding at certain deployment cases, the network densification (densely deployed radio access nodes (AN)) and possibly high gain beamforming become necessary as strategic leverages to meet aforementioned service demands.
In next generation of mobile network systems, it is envisioned that a wide range of applications, services and topology would be served by a common systems. Topologies such as classical cellular, machine type communications, user to users, relays, unlicensed or license assisted networks, etc. are expected to take place and create a diversity of links (not simply access point (AP) to user equipment (UE)) and tend to have a network with less hierarchy.
The wireless network deployments may also have a much larger diversity. Operator networks would typically be well-planned, i.e. with more or less regular distance and coverage, but non-operator managed ones or indoor solutions may have bad or no planning.
The backhauling capabilities of these deployments may also vary from high capacity direct backhaul to wirelessly relayed access points, which impact the coordination potential and media access control (MAC) behavior.
The use cases and topology links present in a next generation of mobile network system have different requirements in term of traffic and quality of service (QoS) needs (e.g. extreme short latency, large throughput, low power, etc.) that have to be handled by the MAC and physical (PHY) layers.
Some of these use cases also create topologies that are more complex and diverse than classical networks. For instance, new links types can occur from UE to UE, AP to AP (e.g. for self-backhauling purposes), multiple links attachments, machine communications, etc.
All these new usages make a single kind of MAC process, which is used to schedule the radio resources, hard to be designed to fit all requirements and high performances. For example, there are scheduling based MAC process used by Long Term Evolution (LTE) and cellular network and contention based MAC process used by the wireless fidelity (Wi-fi) network and wireless local network (WLAN). However, both of them present advantages and disadvantages.
Scheduling Based MAC
LTE and preceding cellular network techniques use scheduling based MAC, a single MAC process managed by APs. In a cell, the AP performs the selection of which resources will be assigned for both uplink (UL) and downlink (DL). UL requests have to be made through physical random access channel (PRACH) by the users to be granted some resources. Assignments of both DL and UL resources are to be transmitted to users prior to payload transmissions.
Cellular and AP-centric approach introduces some difficulty to manage links that are at not directly AP-UE, in particular due to the necessity to choose between Transmit and Receive mode. For example, considering an AP to AP communication, both APs have to make sure that their time division duplex (TDD) modes are correctly configured so that an AP can transmit while the other can receive. Combining multiple types of links or link hierarchy in one cell becomes unpractical to handle with classic scheduling based MAC process.
Additionally, when a UE requests a transmission, the terminal need to quantize and feedback limited information about the desired transmission (buffer status, QoS, etc.) or about its environment (interference situation, bandwidth usage, other active links, etc.). The transmission request from the UE thus introduces some latency. More important issue is that a limited feedback channel could not timely transmit comprehensive information to the AP side.
Hence, a “whole picture” for optimal scheduling is difficult to be available at AP side. Furthermore, if new link type such as UE-UE becomes reality, it is also burdensome for an AP to get information about the UE's duplex status, i.e. transmission (TX) or reception (RX).
Contention Based MAC
Contention based MAC works in a distributed way, where radio resource assignments are decided for each link pair separately. Various schemes exists, such as the simple Listen before Talk, i.e. the transmitter first senses the radio channel for existing transmission, and if possible, transmits directly it's data, or Request to send/Clear to send (RTS/CTS), i.e. the transmitter first transmits a request and the receiver replies with an acknowledgment so as to avoid the hidden-node problem.
Contention based MAC works well when low coordination between cells is needed and is a low complexity solution to allow a diversity of link types. However, it is well known that contention based MAC is suffering high performance losses when heavy loads are in the system if certain coordination or situational parameter adjustment is not available.
In short, since next generation of mobile network is designed to operate in a wide range of frequency bands (from sub 1 GHz to 100 GHz) and shall support a large number of services with fundamentally different requirements (Critical Machine Type Communications (C-MTC), Tactile internet, Mobile broadband), there is some risks that the overall solution becomes very complex. Taking the union of all requirements is a sure receipt for failure. For every use case, we would be able to design a much more suitable system if we only need to fulfill the requirements that are relevant for that particular use case. If the requirements for every use case are to be fulfilled, then there is a high risk that next generation of mobile network ends up being something like a duck: It can swim, it can run, and it can fly. But a shark, a leopard, and an eagle can all do one of those things much better.